GMRS, CB, and AM/FM antennas in close proximity?

irish44j

Well-known member
I prefer not to do roof-mount antennas for the moment (don't want to drill holes, and don't like wires running across the roof), and don't like antennas in front of the cab (personal preference). So I'm considering fabbing up an antenna platform that would sit above my spare tire in the rear (with antennas extending above the roofline by a foot or so, but with the bases protected from branches, etc). I have a dome-style GMRS antenna (one of the Chinese ones similar to the Midland "ghost" antenna), a NGP 2' Firestik for CB, and a 2' AM/FM antenna).

Anyhow, assuming it's about a 18" x 6" platform (steel, if it matters, held there by a steel post), would I have any interference issues having the antennas 6-8" from each other?

Note that I likely would never have the GMRS and CB powered on at the same time, but I do often listen to whatever local radio stations come in wherever I am (yes...I have Spotify too...).

Thoughts?
 

pskhaat

2005 Expedition Trophy Champion
I’ve run GMRS, Quad band 10/6/2/.7, AMPS, and CB/11 all on my bullbar as counterpoise before. Never noticed any problems, though many may cringe.
 
Last edited:

Robert Bills

Explorer
I'm sure that radio experts can come up with a list of legitimate reasons why this should be a last choice, and I'm sure at least one of them can articulate well recognized electronic theory that suggests you will be the victim of a lighting strike the first time you key your microphone.

I would not disagree with any of them, except maybe for the lightning part.

However, in the end you will still have to try it out yourself to see if your particular setup actually causes problems. At low transmit power it may not (and will likely not) be an issue. If you're a high power kind of guy, who knows?

I wouldn't hesitate to experiment.
 

irish44j

Well-known member
I'm sure that radio experts can come up with a list of legitimate reasons why this should be a last choice, and I'm sure at least one of them can articulate well recognized electronic theory that suggests you will be the victim of a lighting strike the first time you key your microphone.

I would not disagree with any of them, except maybe for the lightning part.

However, in the end you will still have to try it out yourself to see if your particular setup actually causes problems. At low transmit power it may not (and will likely not) be an issue. If you're a high power kind of guy, who knows?

I wouldn't hesitate to experiment.
For sure. Worst case I'm out $5 of scrap metal and an hour welding stuff and wiring stuff. The nice part about this is that it would be 100% reversible if I decide later to just drill some holes in different areas of the roof to separate the antennas further.

Also, definitely low-power....just for general use and trails with friends here in the east. There's no open-skies overlanding within 1000 miles it seems, lol.
 

donmontalvo

New member
I run with a Wilson Flex 4' antenna (CB) and a Bingfu 18" antenna (GMRS) side by side on a Teraflex tailgate hinge mount on my 2020 Jeep Wrangler Sport S.

No problems if I run either the CB or GMRS, never ran both at the same time, can't imagine when that would ever happen, I use whichever the group uses. :)
 

DaveInDenver

Expedition Leader
What you're concerned with are two things. One is damaging anything and the second is performance.

Performance (called desensitizing) is a tough one since there's really no way on a vehicle that a transmitting antenna can be far enough away to not interfere with others, especially on the same band. But this is also not usually an issue unless you're a dedicated ham. For example if you have two VHF radios, one perhaps an APRS and a second doing 2m voice. Then there's a real problem. With CB and GMRS the risk for each affecting performance is lower.

As for damage, that's not a simple "this much is definitely enough" answer. Some distance is hopefully enough and more is better. You really don't start getting a lot of reduction in signal strength until 4 wavelengths from the antenna. Anything closer is suspect.

http://www.repeater-builder.com/antenna/separation.html

Things you really need to watch.

One is that the whip of a radio transmitting can't physically touch other whips. That will directly short RF energy into the front end of the receiver which will ruin one or both regardless of frequency. Really do try to avoid this if possible.

The other thing is try to not put the antennas at exact multiples of wavelengths. Frequencies are 26.5-27.5 (CB), 462-467 (GMRS), 420-450 (70cm) and 144-148 (2m). So critical lengths are 25" (GMRS), 27.5" (70cm) and 80" (2m). Try to avoid multiples of these.

And third, use as little power as possible as often as you can.

To put some numbers to it. A 50 watt transmitter is +47 dBm and 5 watts is +37 dBm (a reduction of 10 dBm). Both far exceed the damaging point for most radios. A typical absolute max input for a radio is about +17 dBm and the ARRL during radio tests limits the maximum input to +10 dBm. And this is before antenna gain.

So strictly speaking for 2m VHF ham at 50 watts a receiving radio near-by radio will need 37 dB of attenuation, which will occur at about 67 feet in free space without any antenna gain, to be considered safe. And 5 watts will require 27 dB and still be 20 feet.

I know this will be routinely ignored. Heck, I'm grossly violating this on my truck since I have two antennas about 20" apart on the roof of my truck. That's only giving maybe 5.5 dB of attenuation on VHF and 15 dB on UHF and is a multiple on 2m. So do not do what I did.
 
Last edited:

colodak

Adventurer
I run a CB (Cobra WX 75), GMRS (Midland) and a 2/70 (Yaesu) handheld with a mag mount antenna. My CB antenna is a fender mount, the GMRS and handheld are mag mounts set on the roof, they are about 1m/3ft apart, on the GMRS repeater channels, there is definite bleed over to the Ham, I asked about this on radio reference once, the feedback I got was not very helpful, a few suggestion were to space the antenna further apart, turn one off when using the other. My current plan, once I free up the cash, is to leave the GMRS antenna on the cab roof, then move my 2/70 antenna to a mount on my topper and install a fixed radio in the vehicle, I'd like to try and keep a 1.5m to 2m separation between antenna if possible.

Recently I talked to a guy with a Ram 2500 crewcab short bed, he's got 4 antenna on his, CB on the fender, GMRS on his bumper hoop, a 2/70 on a folding antenna mount in the middle of a roof rack on the cab and a 6m on a folding mount on his topper. He said that he had similar problems, and would turn off the HAM's when using the GMRS.
 

DaveInDenver

Expedition Leader
FWIW @colodak powering off is mostly a false sense of security. It'll protect some of the highly sensitive circuits but the actual front end protection is vulnerable whether the radio is on or off. That protection is designed to shunt gross abuse, static (ESD), lightning, very high RF fields, etc. Once that's failed then you leave the receiver itself exposed to damage, so the original failure is a latent failure that can casade into totally dead radio down the road.
 

CaliMobber

Adventurer
You will be fine, should be no real issues. All those freq are far from each other and should not cause either any real issues. Go for it!
 

DaveInDenver

Expedition Leader
Was doing some testing with my truck roof antennas. I normally have a Larsen NMO2/70 dual band (for the regular voice radio) and a STI-CO Flexiwhip 1/4λ cut for 2m (for an APRS radio) on the roof. They are very close together at about 20".

I calculated 5.5 dB of isolation and am actually achieving 14.9 dB at VHF. This is tremendously bad, no doubt about it. I knew it would be.

twin_roof_whips_dualband-vhf_mid.jpg

On UHF the isolation to a VHF antenna just 20" away isn't a ton better at 28 dB. I replaced the NMO2/70 with a Larsen NMO27 that I use for CB. This shows 54 dB of isolation when the VHF antenna is the aggressor and the CB antenna the victim.

So the take-away is definitely do not put same band antennas close (duh, right?).

GMRS to 2m VHF is marginal at low power. Using 5W would see about +9dBm in the other radio's front end but would not be wise to do with high power (50W).

ETA: Single band radios (a 2m ham next to a GMRS antenna) might be more compatible since they would filter out of band more aggressively. A dual band 2m/70cm ham radio wouldn't tolerate a GMRS antenna in close proximity.

CB to VHF/UHF is low risk.
 
Last edited:
Top